The present invention relates to organic polymers known as polyiminocarbonates and to methods of synthesizing such polymers.
Deficiencies in the mechanical, physical and toxicological properties of degradable plastic materials suggested for use as tissue compatible materials has lead to an international research effort directed towards development of degradable polymers for biomedical uses. Such degradable polymers also address the growing public concern about the accumulation of non-degradable plastic materials in the environment.
In the field of bio-medicine, Kohn et al., U.S. Pat. No. 4,638,045 describe the synthesis of non-peptide poly (amino acid) bioerodable polymers useful for controlled release applications such as delivery of a wide variety of biologically and pharmacologically active compounds. The polymers were prepared by polymerizing --L-- amino acids or dipeptides by non-amide bonds, such as iminocarbonate bonds, located on the amino acid side chains.
Kohn et al., Bio-materials, 7, 176-182 (1986) further disclose the usefulness of polyiminocarbonates as potential bio-materials.
Polyiminocarbonates are structurally related to polycarbonates. The polyiminocarbonates have imino groups in the places normally occupied by carbonyl oxygen in the polycarbonates. Thus, the polyiminocarbonates have linkages according to the formula: ##STR1##
This imparts a significant degree of hydrolytic instability to the polymer. The polyiminocarbonates have desirable mechanical properties akin to those of the corresponding polycarbonates.
Compared to polycarbonates, little work has thus far been published on the synthesis of polyiminocarbonates. Hedayatullah, Bulletin De La Societe Chemique De France, 2, 416-421 (1967), in the first reported synthesis of a polyiminocarbonate, reacted aqueous solutions of various chlorinated diphenolate sodium salts with cyanogen bromide dissolved in methylene chloride. Hedayatullah only reported the melting points and elemental analysis of the obtained products, and it is not easy to determine with certainty to what extent truly polymeric materials were prepared. Later, Schminke et al., U.S. Pat. No. 3,491,060 reported that Hedayatullah's procedure yielded only oligomers with molecular weights below 5,000 daltons. The '060 patent describes synthesis of polyiminocarbonates by solution polymerization using an equimolar mixture of aromatic dicyanate and diphenol in solution. The '060 patent recites a broad range of solvents and catalysts, and asserts that the solution polymerization procedure provides "polymers with molecular weights up to about 60,000."
However, no method has been disclosed to date for the synthesis of polyiminocarbonates having molecular weights in excess of 60,000 daltons. Moreover, no method has been disclosed to date for synthesis of the dipeptide-based polyiminocarbonates as disclosed in the '045 patent having molecular weights in excess of 20,000 daltons. Higher molecular weight iminocarbonates are desired because higher molecular weight polymers generally provide better mechanical properties.
Thus, there have been unmet needs heretofore for improved methods of synthesizing polyiminocarbonates and for improved polyiminocarbonates.